1. Field of the Invention
This invention relates to vehicle fuel vapor loss control systems in general, and specifically to such a system of the type that uses a liquid seal as the primary means to prevent the escape of fuel vapors up the filler pipe to atmosphere.
2. Description of the Related Art
Production vehicle fuel vapor loss control systems have for some time included a vapor storage canister and the associated vapor lines from the fuel tank to the canister necessary to control the vapors that form in the tank as it sits, typically referred to as the diurnal losses. Before that, diurnal losses were generally vented to atmosphere. Other significant sources of vapor loss to atmosphere are the fuel vapors displaced from the tank as it is filled, referred to as fuel fill vapors, and the pressurized vapors that form in the filler pipe when it is closed and which are released as the cap is removed, referred to descriptively as the "puff loss." Current production systems generally just vent fuel fill vapors and puff losses to atmosphere, but more stringent air quality standards currently under debate may require that these, too, be controlled. Of the two other sources of vapor loss, fuel fill vapors are by far the largest in volume, so newly proposed vapor loss control systems are directed to controlling them at least. More far reaching proposed designs seek to control puff losses as well, however.
A common feature of the new systems proposed to recover fuel vapors is some sort of seal to block their exit to atmosphere. At the same time, a relatively free exit from the tank must be provided for fuel fill vapors to avoid pressure build up in the tank. Two basic sealing means show up frequently among the various proposed designs. Most common is a mechanical seal near the top of the filler pipe that tightly surrounds the nozzle as it is inserted to block the filler pipe. An exit port and vapor line through the filler pipe below the mechanical seal provide an exit path to the canister for the fuel fill vapors. Generally, some selective valve means is also provided to block the vapor exit path to the canister when the filler pipe is closed, and to open it only when the cap is removed or when the nozzle is inserted. A good example of such a mechanical seal system may be seen in U.S. Pat. No. 4,630,749 to Armstrong et al, assigned to the assignee of the subject invention. Less common is what has come to be known as a liquid seal system, which uses no mechanical seal to physically contact the nozzle. Instead, sealing is provided by the fuel from the dispensing nozzle. A liquid trap may be built into the filler pipe, or the force of the stream of fuel may itself be used to prevent the escape of vapors from the tank. Examples of several types of liquid seals may be seen in SAE Technical Paper #861551, published in Oct. of 1986.
The main advantage of a liquid seal system is that there is no rubbing seal around the nozzle to wear, a seal that may see rough use as different sized, worn, bent, and heavily leaned upon nozzles are inserted through it at varying angles. The lack of a convenient blocking seal around the nozzle is also a potential drawback, however. A path must still be provided for routing fuel fill vapors to the canister, and a mechanical nozzle seal can be used to help create that exit path. Likewise, the insertion of the nozzle through a seal can keep puff loss vapors from escaping the filler pipe until the nozzle triggers and opens a downstream port to a vapor vent line. Still, if a liquid seal system could be devised that would handle fuel vapors as well as a mechanical seal system does, it could prove very useful.